VMS Help CMS, MODIFY, RESERVATION, Qualifiers, /IDENTIFICATION_NUMBER=n  *Conan The Librarian (sorry for the slow response - running on an old VAX)

    Specifies which reservation is to be modified. This qualifier
    is required when you have multiple reservations of the same
    generation of an element. /IDENTIFICATION_NUMBER can be used
    instead of /GENERATION when you have multiple reservations. Use
    the SHOW RESERVATIONS command to determine the identification
    number of each reservation. The identification number appears
    before the user name.